Nitride materials have applications in many fields of use because of their wide range of desirable physical, optical, and electronic properties. Many applications make use of the relative chemical inertness of these materials, which are typically stable at high temperatures and resistant to corrosion in acids. See, for example, Maboudiani and Carraro, Annual Review of Physical Chemistry 35-54, 55 (2004), and Strite and Morkoc, Journal of Vacuum Science and Technology B 1247-1266, 10 (1992). However, there are numerous applications where it is desirable to bond molecules to a nitride surface in order to provide the surface with specific chemical, physical, mechanical, and/or optical properties or functions (hereafter referred to as a functionalized surface). For applications where the functionalized surfaces are subsequently exposed to chemically reactive environments, including but not limited to ambient air, solvents, aqueous solutions, and biological matrices in vivo, covalent bonding directly to the surface would be particularly desirable. Some applications where functionalization via covalent bonding directly to a nitride surface would be desirable include but are not limited to chemical and biological sensors, microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS), and biocompatible materials and devices. See, for example. D. Baselt. et al. Biosensors and Bioelectronics 731-739. 13 (1998); Craighead, Science 1532-1535, 290 (2000); Dion, Biomaterials 169-176, 14 (1993); Xu and Shi, Biomedical Scientific Instrumentation 585-589, 33 (1997).
Nitride materials exposed to air or water will naturally oxidize. Therefore, there is much prior art describing covalent coupling reactions to surface oxides, particularly for silicon nitride. See Holloway et al., Journal of the Electrochemical Society 723, 123 (1976) and Matsuo et al., Sensors and Actuators 77, 1 (1981). On a clean, stoichiometric silicon nitride surface, Si3N4, each nitrogen atom is bonded to three silicon atoms. Thus a complete surface hydrolysis reaction would be:Si3N+2H2O→SiNH2+2SiOH.
Si3N4 surfaces hydrate completely into primary amines and silanols. See Harame, et al., IEEE Transactions on Electron Devices 1700, 34 (1987). Other nitrides, Linder the appropriate conditions, should also undergo surface hydrolysis to yield an amine+alcohol-terminated surface. Functionalization of silicon nitride surfaces has been performed almost exclusively using silane chemistry for covalent attachment through the silanol groups, most notably in biosensing applications. See Gao. et al . . . Sensors and Actuators B 38-39, 38 (1997) and Manning et al., Langmuir 395, 21 (2005). In general, the silane chemistries are extremely sensitive to pH, temperature, and relative humidity. The sensitivity of silane chemistries to these conditions make these methods difficult to perform consistently, causing the densities of the functional molecules to be unreliable and variable across each surface, and potentially leaving variable areas of the surface uncoated. These shortcomings are undesirable in many applications.
Although there are many coupling chemistries suitable for covalent bonding to an oxidized nitride surface, via a silanol for example, direct coupling to a surface amine might have advantages. Secondary functionalization through surface amines, as opposed to surface silanols, is much less sensitive to reaction conditions and environmental factors. Many molecules commonly used to functionalize surfaces intrinsically possess amine-reactive functionalities, especially in biomolecular applications. For example, aldehyde-amine reactions proceed to an appreciable extent over a range of pH values, including the physiological 6.8-7.8 range that many biomolecules, especially proteins, require to preserve their activity. The aldehyde-amine reaction is also much less dependent on environmental factors, including temperature and relative humidity, which can be difficult to control in basic laboratory or production settings. Similarly wide ranges of reaction conditions exist for other amine-reactive chemical groups, including, but not limited to, carboxylic acids, alcohols, sulfhydryls, and epoxides. A person skilled in the art of chemistry, biochemistry, or a related field would recognize there are many such methods.
Functionalization of silicon nitride surfaces could proceed via covalent chemistry that indiscriminately reacts with surface silanols and primary amines present at an unknown and uncontrolled surface density as a result of unintended chemical reactions that may occur during cleaning, processing, or storage. See Karymov. et al., Sensors and Actuators B 324, 29 (1995) and Karymov, et al. Langmuir 4748, 12 (1996). It is important to note that though the bonding may be reported as direct to the nitride surface, coupling via such native surface amines has only been presumed based on the assumption of stoichiometric surface hydrolysis in air or water, and the functional molecules of interest have been coupled to the surface through an intermediate chemical linker for increased stability.
Therefore, there is a need in the art for a method to controllably and reliably active nitride surfaces for stable amine-reactive chemistry so that the surfaces can be functionalized.